Fractures as Advective Conduits at the Earth-Atmosphere Interface

Our planet is active and changing. Gas exchange across the earth-atmosphere interface is an essential component, impacting a number of important earth and atmospheric processes, including global water cycle, greenhouse gas emissions, solute precipitation within the vadose zone, and the evolution of karst terrain. Both diffusive and advective mechanisms contribute to net gas exchange at different time and spatial scales. This chapter discusses advective gas transport via vadose zone fractures driven by surface wind and vadose zone thermal gradients. Two critical aspects of thermal-driven fracture ventilation are evident: (1) Thermal gradients in the top meter of the vadose zone are sufficient to trigger deep penetration of atmospheric air and rapid replacement of fracture air, and (2) Advection by this mechanism occurs only during cooler nighttime hours. On a seasonal scale, deeper vadose zone temperature controls the proportion of the day during which venting is active, with more venting occurring during winter. Wind-driven convection also generates fracture air venting; however, venting does not penetrate as deeply into the vadose zone and its timing and duration depend upon local weather patterns. The work presented here represents the beginning of a quantitative understanding of vadose zone venting via fractures by these two mechanisms.